MONDAY, June 28 (HealthDay News) -- A new gender-specific formula for a woman's peak heart rate better predicts the risk of heart-related death, a new study finds.

A simple formula -- 220 minus age -- has long been used to calculate peak heart rates for women and men during exercise. But that formula was based on studies of men. The new formula developed for women is 206 minus 88% of age.

At age 50, the peak heart rate for a man is 170 beats per minute. But the new formula suggests a peak rate of 162 beats per minute for a 50-year-old woman. Many women and men use their peak heart rate multiplied by 65% to 85% to calculate their maximum heart rate when exercising.

The new formula is based on an analysis of 5,437 healthy women aged 35 and older (average age 52) who took part in the St. James Women Take Heart Project, launched in the Chicago area in 1992.

"Using the [original] formula, we are more likely to tell women they have a worse prognosis than they do. With this gender-specific formula, our risk predictions in women will be more accurate," study author Dr. Martha Gulati, an assistant professor of medicine and preventive medicine at Northwestern University's Feinberg School of Medicine in Chicago, said in an American Heart Association news release.

"Women are not small men. There is a physiologic response in women that is different from men," she noted.

Using the new formula and adjusting for factors such as heart risk, researchers found that women who scored less than 0.8 on a measure known as the chronotropic index -- which measures the relationship between heart rate and oxygen uptake during exercise -- were 30% more likely to die prematurely of any cause.